Effective emission control of internal combustion engine exhaust gases with a catalytic converter requires precise control of the air/fuel ratio supplied to the engine cylinders. For this purpose, it is customary to install an oxygen sensor in the engine exhaust pipe, and to use the sensor output as a feedback signal for closed-loop fuel control. Commonly, the exhaust gases of several cylinders are combined in an exhaust manifold, with a single oxygen sensor is positioned near the outlet of the exhaust manifold, and an average reading of the oxygen sensor is used as a common feedback signal for controlling the fuel supplied to each of the several cylinders.
The above-described control technique assumes a uniform air and fuel distribution among the several cylinders. However, there are frequently significant variations in air and fuel distribution among different cylinders, due to engine hardware design variations and fuel injector performance variations, for example. These variations cause the actual air/fuel ratio to significantly depart from the target air/fuel ratio, especially under transient conditions. For this reason, it has been proposed to individually control the fuel supplied to each engine cylinder; see, for example, the U.S. Pat. No. 5,651,353 to Allston, issued on Jul. 29, 1997, and assigned to the assignee of the present invention. Some systems of this type utilize multiple oxygen sensors for developing air/fuel ratio feedback signals unique to each cylinder. Some systems utilize a single oxygen sensor, but variations in the transport delay time between a cylinder exhaust port and the oxygen sensor make it difficult to correlate the sensor readings with a giver cylinder. In certain instances, transport delay variations are empirically determined for a given engine design, and stored as a fueling calibration. In other instances, mathematical models of the engine and exhaust system are used to compute individual air/fuel ratio feedback signals. The computation approach is often impractical due to through-put requirements, and both approaches suffer inaccuracy due to component variation (such as injector performance variations) and degradation over time. Further, neither approach can adapt to engine hardware changes, such as installation of a different exhaust manifold or re-location of the oxygen sensor.